1. Field of the Invention
The present invention relates to a self-diagnostic device and a self-diagnostic method for performing a diagnosis as to whether or not an acceleration or angular-velocity sensor is in a normal status. The present invention further relates to an acceleration or angular-velocity sensor provided with such a self-diagnostic device, and a method of initially setting the acceleration or angular-velocity sensor.
2. Description of the Background Art
An acceleration and angular-velocity sensor is widely used in controlling of an air bag device of a vehicle such as a motor vehicle. For example, an air bag control device disclosed in Japanese Patent Laying-Open No. 2006-056441 includes a main acceleration sensor, and a safety acceleration sensor disposed at a position different from that of the main acceleration sensor, so as to prevent malfunction and ensure reliable activation.
In the air bag control device described in this document, the main acceleration sensor detects acceleration and outputs an acceleration signal, and a microcontroller integrates the acceleration signal received from the main acceleration sensor. When an integration value of the acceleration exceeds a prescribed value because the acceleration is increased in a deceleration direction owing to a collision of the vehicle, the microcontroller determines that the vehicle has collided, and outputs a trigger signal at a high level to an AND circuit.
Further, an acceleration detection output of the safety acceleration sensor is A/D (Analog to Digital)-converted by an A/D converter, and inputted to an operational circuit. The operational circuit integrates the A/D-converted output, and inputs it to a determination circuit. The determination circuit then compares the integrated output received from the operational circuit with a threshold level, and when the integrated output exceeds the threshold level, outputs a signal at a high level to the AND circuit. At this time, when a collision detection signal is inputted to the AND circuit from the microcontroller, the AND circuit opens its gate and transmits a signal to the base of a transistor, so that the transistor is turned on, a current is supplied to a squib from a power source, and the air bag is activated.
Further, Japanese Patent Laying-Open No. 2007-245829 discloses an air bag control device that detects a rollover, which represents a turnover of a vehicle. The air bag control device in this document captures an output value of an angular-velocity sensor, integrates the output value for prescribed time, and compares the generated integration value with a set determination threshold value, so as to determine whether or not an offset correction is correctly performed. Based on the premise that the offset correction is correctly performed, the air bag control device determines whether or not a rollover occurs. If the air bag control device detects a rollover, it supplies a current to a squib, which is identified as a heating resistor body for inflating the air bag, to activate the air bag.
Generally, in the acceleration or angular-velocity sensor, a self-diagnosis (primary check) for determining the presence or absence of a failure of the sensor is performed at power-up or the like. Specifically, a pseudo signal that corresponds to an acceleration or angular-velocity signal is applied to a sensor element, and based on a signal level of a sensor output, which signal level changes at that time, a primary check is conducted at power-up or the like.
Japanese Patent Laying-Open No. 2001-304871 discloses an abnormality diagnostic device for an angular-velocity sensor, capable of performing a correct self-diagnosis even if unexpected angular velocity is inputted thereto at the time of the self-diagnosis. In the abnormality diagnostic device, a signal level of an angular-velocity output, which is obtained in the state that a rotation pseudo signal is not applied to the sensor element, is held by a hold circuit in a primary check circuit. Based on the held signal level, a threshold value is variably set. A window comparator compares the set threshold value with a signal level of an angular-velocity output obtained in the state that a rotation pseudo signal is applied to the sensor element, and based on the results, performs a diagnosis as to the presence or absence of abnormalities.
Further, Japanese Patent Laying-Open No. 2003-262648 discloses a technique for suppressing an adverse effect on an offset correction operation, which adverse effect is caused by a self-diagnostic output.
According to this document, when a self-diagnostic circuit inputs a rectangular-wave self-diagnostic output to an acceleration sensor so as to perform a self-diagnostic check on an acceleration sensor, the acceleration sensor outputs a differential waveform. Thereby, variations appear in an output of a low-pass filter processing circuit for offset correction. Similar variations also appear in a corrected output obtained through an offset correction operation at an operational unit in a CPU (Central Processing Unit), so that the corrected output requires longer time for converging to a zero point, and zero point accuracy is lowered. In order to avoid this phenomenon, the offset correction operation is performed in the CPU prior to the self-diagnostic check (i.e. before the rectangular-wave self-diagnostic output is inputted to the acceleration sensor), or preferably concurrently with power-up.
The self-diagnosis as to whether or not the acceleration or angular velocity sensor normally operates is performed while the acceleration or angular velocity sensor is being operated. Therefore, if the determination as to whether or not the sensor is in a normal status is made in the state that the sensor main body receives a test signal for providing pseudo acceleration or angular velocity, and additionally receives unexpected acceleration or angular velocity externally exerted thereon, an incorrect diagnostic result is inevitably outputted.
Japanese Patent Laying-Open No. 2001-304871 described above provides one method for addressing this problem. In the method in this document, however, a signal level of a sensor output obtained in the state that a pseudo signal is applied to the sensor element is corrected based on a signal level of the sensor output obtained in the state that the pseudo signal is not applied to the sensor element. Therefore, if an instantaneous impact is exerted on the sensor, the acceleration or angular velocity externally exerted on the sensor may differ between the case that the pseudo signal is applied and the case that the pseudo signal is not applied, and hence a diagnosis may not be performed correctly.